1. Field of the Invention
The present invention relates to a procedure for irreversibly transferring a diffraction grating, such as a stamped hologram, onto a substrate, such as a document or a product to be secured, such as for example currency or banknotes, administrative documents, fiduciary documents, identification documents, and credit cards.
2. Description of the Related Art
The prior art includes essentially two methods for transferring an optical component such as a diffraction grating or a hologram stamped onto the surface of a document or of a product made of paper or of plastic, i.e.:
Heat marking; and PA0 The application of a label.
Transfers that involve the application of a label are not readily compatible with automated marking methods. Furthermore, labels can easily be removed from their initial substrate and re-used on another substrate--a procedure that is contrary to the securing of the substrate.
Labels have been proposed that include an embrittlement layer that causes the localized separation of the layers in the event of an attempt to remove the label. However, it is difficult to make the label sturdy enough to withstand everyday handling of the document to which it is applied, and at the same time to ensure the effectiveness of the destructive separation in the event that an improper attempt is made to remove the label.
In order to remedy these disadvantages, the prior art, as disclosed for example in British patent No. GB-2 181 993 and in British patent No. GB-1 129 739, has proposed a marking procedure, and particularly a heat-marking procedure, that involves preparing a multi-layer transfer film consisting of a very thin supporting film substrate, usually less than 25 microns thick; a heat-reactivatable adhesive; a transparent metallized or reflective layer deformed by stamping in order to carry the optical image; a protective film overlay; and a detachment layer.
The transfer is accomplished through the application of heat and strong pressure to the film with the aid of a tool that has the same shape as the element to be transferred. The heat locally reactivates the thermal adhesive, and the active layers of the optical component are glued in accordance with the contour of the tool.
The carrier film is then removed. The layers that form the optical component break along the line surrounding the periphery of the glued surface, in accordance with the shape of the transfer tool. The glued surface then separates from the film substrate at the detachment layer.
This transfer procedure is satisfactory for optical components that consist of layers that are thin enough to ensure a clean break and separation of the active component along the line separating that component from the region containing the reactivated adhesive.
However, this procedure is not compatible with the transfer of optical components that consist of thick layers, or with the use of transfer films that include a thick supporting film substrate.
Moreover, the transfer tool is specific to the element to be transferred, inasmuch as its shape must correspond exactly to the shape of the optical component that is to be deposited onto the substrate. Furthermore, for continuous transfers, the optical image carried by the film must be positioned automatically under the heating tool. This procedure requires the use of optical registration systems located on the transfer matrix.
Finally, the choice of adhesive is critical, because the adhesive in question must allow a clean break along the edge of the glued region.